Separately stamped metal sheets are secured to each other to form body panels in automotive applications using induction bonding. An electromagnetic field is applied to the metal sheets, generating an eddy current which heats the metal sheets to allow the adhesive to cure such that the metal sheets are bonded together. The temperature of the body panels needs to be controlled during bonding since the temperature affects the strength of the adhesive bond between the two metal sheets. Therefore, the temperature of the metal sheets requires accurate monitoring.
Currently, the temperature of the metal sheets in the induction bonding process is difficult to monitor due to the problems associated with measuring temperatures on strong electromagnetic fields. Conventionally, the temperature of the metal sheets is typically measured using thermocouples or infrared sensors. When using thermocouples, the eddy current generated by the electromagnetic field affects the current of the thermocouple, thereby providing inaccurate temperature readings.
When using infrared sensors, contamination such as dust, mill oil and scratches, affects the sensor output as well as emissivity and ambient reflections. Mill oil is used in the stamping process of the metal sheets to prevent tearing. The mill oil is not removed prior to bonding the metal sheets. This mill oil affects the emissivity of the metal sheets, which is a primary source of errors in infrared temperature measurement. The emissivity of a surface is the percentage of the surface that emits. The more a surface emits, the more accurate the temperature reading.
A temperature-measuring device is disclosed in U.S. Pat. No. 4,396,791 issued to Mazzoni. Mazzoni discloses a cylindrical probe adapted for tension adjustment for measuring the temperature of a laminator platen. A thermocouple is attached to a thermally conductive body of the probe. The Mazzoni device measures temperature via a thermocouple, which is subject to error when placed in an electromagnetic field.
A temperature-measuring device is also disclosed in U.S. Pat. No. 3,626,758 issued to Stewart et al. Stewart et al discloses a temperature sensor for measuring temperature in a particular region. The Stewart et al device includes a conical tip to be inserted into the region to be measured. The conical tip of the Stewart et al device is not suitable for measuring the temperature of a solid surface.
A problem of the known prior art, therefore, is that temperature measurements of metal sheets during induction bonding are difficult to obtain accurately due to the strong electromagnetic field, mill oil and the changing surface emissivity of the metal sheets.